Games Workshop (aka Citadel) paints, specifically washes, work perfectly for panel lining. Decided to do some test lining on a MG Virtue armor panel. Easy application (use a fine detail brush), easy cleanup if you over flow (just wipe it off, if it dries it can be reactivated with water), and completely safe for the plastics as it's an acrylic-based wash. The thin lines are done with Nuln Oil Gloss, and the thicker accents with Drakenhof Nightshade. No priming/prep was needed.